The invention relates to a novel thermal spray process for the deposition of coatings with a graded or layered composition on a substrate and the coated articles produced thereby. More particularly, the invention relates to feeding at least two coating materials to a thermal spray device and continuously or intermittently changing the composition of the deposited coatings by changing the thermal spray operating parameters. The change in the composition of the coating during deposition creates a graded or layered coating structure.
The family of thermal spray processes includes detonation gun deposition, high velocity oxy-fuel deposition and its variants such as high velocity air-fuel, plasma spray, flame spray, and electric wire arc spray. In most thermal coating processes metallic, ceramic, cermet, or some polymeric materials in powder, wire or rod form is heated to near or somewhat above its melting point and droplets of the material accelerated in a gas stream. The droplets are directed against the surface of a substrate to be coated where they adhere and flow into thin lamellar particles called splats. The coating is built up of multiple splats overlapping and interlocking. These processes and the coatings they produce have been described in detail in the following: xe2x80x9cAdvanced Thermal Spray Deposition Techniquesxe2x80x9d, R. C. Tucker, Jr., in Handbook of Deposition Technologies for Films and Coatings, R. F. Bunshah, ed., Second Edition, Noyes Publications, Park Ridge, N.J., 1994, pp. 591 to 6421; xe2x80x9cThermal Spray Coatingsxe2x80x9d, R. C. Tucker, Jr. in Handbook of Thin Films Process Technology, Institute of Physics Publishing, Ltd., London, 1995; and xe2x80x9cThermal Spray Coatingsxe2x80x9d, R. C. Tucker, Jr., in Surface Engineering ASM Handbook, Vol. 5, ASM International, Materials Park, Ohio, 1994, pp 497-509.
In virtually all of the thermal spray processes two of the most important parameters controlling the structure and properties of the coatings are the temperature and velocity of the individual particles as they impact on the surface to be coated. Of these, the temperature of the particles is of greatest importance relative to the present invention. The temperature the particles achieve during the deposition process is a function of a number of parameters including the temperature and enthalpy (heat content) of the process gases, the specific mechanisms of heat transfer to the particles, the composition and thermal properties of the particles, the size and shape distributions of the particles, the mass flow rate of the particles relative to the gas flow rate, and the time of transit of the particles. The velocity the particles achieve is a function of a number of parameters as well, and some of these are the same as those that affect the particle temperature including the composition, velocity and flow rate of the gases, the size and shape distributions of the particles, the mass injection rate and density of the particles. Thus the thermal gas dynamics characteristics of the thermal spray process determine the quality of the resulting coating.
In a typical detonation gun deposition process, a mixture of oxygen and a fuel such as acetylene along with a pulse of powder of the coating material is injected into a barrel, such as a barrel of about 25 mm in diameter and over a meter long. The gas mixture is detonated, and the detonation wave moving down the barrel heats the powder to near or somewhat above its melting point and accelerates it to a velocity of about 750 m/sec. The molten, or nearly molten droplets of material strike the surface of the substrate to be coated and flow into strongly bonded splats. After each detonation, the barrel is generally purged with an inert gas such as nitrogen, and the process repeated many times a second. Detonation gun coatings typically have a porosity of less than two volume percent with very high cohesive strength as well as very high bond strength to the substrate. In the Super D-Gun(trademark) coating process, the gas mixture includes other fuel gases in addition to acetylene. As a result there is an increase in the volume of the detonation gas products which increases the pressure and hence greatly increases the gas velocity. This, in turn, increases the coating material particle velocity which may exceed 1000 m/sec. The increased particle velocity can result in an increase in coating bond strength, density, and an increase in coating compressive residual stress. In both the detonation gun and Super D-Gun coating processes, nitrogen or another inert gas can be added to the detonation gas mixture to control the temperature of the detonated gas mixture and hence the powder temperature. A number of parameters can be used to control both the particle temperature and velocity including the composition and flow rates of the gases into the gun.
In high velocity oxy-fuel and related coating processes, an oxygen, air or another source of oxygen is used to burn a fuel such as hydrogen, propane, propylene, acetylene or kerosene in a combustion chamber and the gaseous combustion products allowed to expand through a nozzle. The gas velocity may be supersonic. Powdered coating material is injected into the nozzle and heated to near or above its melting point and accelerated to a relatively high velocity, such as up to about 600 m/sec. for some coating systems. The temperature and velocity of the gas stream through the nozzle, and ultimately the powder particles, can be controlled by varying the composition and flow rate of the gases or liquids into the gun. The molten particles impinge on the surface to be coated and flow into fairly densely packed splats that are well bonded to the substrate and each other.
In the plasma spray coating process a gas is partially ionized by an electric arc as it flows around a tungsten cathode and through a relatively short converging and diverging nozzle. The partially ionized gas, or gas plasma, is usually based on argon, but may contain, for example, hydrogen, nitrogen, or helium. The temperature of the plasma at its core may exceed 30,000 K and the velocity of the gas may be supersonic. Coating material, usually in the form of powder, is injected into the gas plasma and is heated to near or above its melting point and accelerated to a velocity that may reach about 600 m/sec. The rate of heat transfer to the coating material and the ultimate temperature of the coating material are a function of the flow rate and composition of the gas plasma as well as the torch design and powder injection technique. The molten particles are projected against the surface to be coated forming adherent splats.
In the flame spray coating process, oxygen and a fuel such as acetylene are combusted in a torch. Powder, wire or rod is injected into the flame where it is melted and accelerated. Particle velocities may reach about 300 m/sec. The maximum temperature of the gas and ultimately the coating material is a function of the flow rate and composition of the gases used and the torch design. Again, the molten particles are projected against the surface to be coated forming adherent splats.
Thermal spray coating processes have been used for many years to deposit layered coatings. These coatings consist of discrete layers of different composition and properties. For example, the coating may be a simple duplex coating consisting of a layer of a metal alloy such as nickel-20 chromium (compositions herein are in weight percent unless otherwise noted) adjacent to the substrate with a layer of zirconia over it. In this case the undercoat of nickel-chromium may be used to enhance the mechanical or thermal shock resistance of the coating system or to protect the substrate from corrosion. An increase in mechanical or thermal shock resistance may be achieved by adding a third layer of coating consisting of a mixture of nickel-chromium and zirconia between the pure nickel-chromium and zirconia layers. Alternatively, perhaps even better thermal or mechanical shock resistance could be achieved by using two or more intermediate layers, each with an increasing amount of zirconia, thus approximating a continuously graded structure. Recently some graded coatings have been referred to as xe2x80x9cfunctionally gradedxe2x80x9d coating systems.
Prior to the present invention the general means of creating graded coating structures was to intermittently change the powder, wire or rod coating material composition being fed to the thermal spray device or devices if more than one was used to deposit the various layers. The coating deposition parameters and in some cases the coating thermal spray device were changed with each layer to obtain the desired coating structure of the particular layer composition. In most cases this meant that the coating process had to be stopped; the powder, wire, or rod feeders and composition changed; the deposition parameters such as electrical power, gas flows, and gas compositions changed; the process restarted; and new coating qualified before the coating of the substrate could be continued. All of this added very substantial time and cost to the coating process. Moreover, the time between coating layers generally tends to decrease the bond strength between layers and the over-all strength of the coating system.
It is an object of the present invention to provide a novel process for the production of thermal spray coatings that are graded in composition and/or properties.
It is a further object of the present invention to provide novel coatings that are graded in composition and/or density properties produced by the said novel process.
It is also a further object of the present invention to provide articles with graded coatings produced using the novel process of the invention.
It is a further object of this invention to provide a process for the deposition of graded coatings with greater cohesive strength than can be achieved using the multiple layer coatings of the present art.
It is also a further object of this invention to provide graded coatings with greater cohesive strength than that of the graded coatings of the present art. The higher cohesive strength of the coatings of this invention is believed to be the result of the smoother transition in composition and properties achieved with the process of this invention and the minimal time between the deposition of the layers of the coating.
The invention relates to a process for producing a graded thermal spray coated layer on a substrate comprising the feeding of a mixture of at least two coating materials to a thermal spray device and varying at least one of the deposition parameters of the thermal spray device during the deposition operating thereby varying the composition of the deposited coating material to produce a graded coated heterogeneous layer on the substrate. The thermal spray device for the process of this invention has parameters that can control or monitor the temperature of the depositing coating material and the velocity of the coating material particles.
The invention also relates to the deposition by means of the novel coating process of this invention, of unique coating structures with smoothly varying gradations in composition and/or density properties. Since the changes in deposition parameters can be made while the coating is being continuously deposited, the gradation or changes in composition and/or density properties are very smooth. If the coating is being continuously deposited, the gradation or changes in composition and properties are very smooth. If the coating is deposited without moving the gun or torch and the substrate is stationary as well, the coating gradation will be continuous; i.e., without discrete changes as a function of thickness. In most cases, however, the coating device and substrate can be moved relative to each other and the coating is deposited in multiple layers. Using the process of this invention, each layer may be slightly different than the preceding or succeeding layer. The time between layers is only dependent on the size of the substrate and the traverse rate (the relative rate of motion between the coating device and the substrate), since coating is being deposited continuously by the coating device. The difference between layers is a function of the rate of change in deposition parameters and the traverse rate. The smoothness of the gradation is then a function of the thickness of the individual layers which can be made very thin. The total thickness of the coating and of each zone is a function of the requirements of the application. The total thickness of the coating is typically in the range of 100 to 500 microns, but may be thicker or thinner if it is necessary to satisfy the specific requirements of the application.
This invention also relates to articles with the graded coatings of this invention. Such articles include those requiring coatings with graded properties to enhance the coating""s mechanical, thermal, or electrical properties. As illustrative, but not limited, examples of these coated articles include (a) those that require a very hard, brittle surface such as an oxide or carbide graded to a tough, ductile metallic layer adjacent to the substrate to provide more impact resistance and higher bond strength of the coating to the substrate; (b) those that require a thermal barrier outer layer such as an oxide graded to a metallic layer adjacent to the substrate that provides oxidation resistance and thermal shock resistance; and (c) those that require a wear and corrosion resistant outer surface graded to an oxide layer adjacent to the substrate for electrical resistance.
As used herein, a graded coated layer shall mean at least one layer comprising a mixture of at least two coating materials in which at least one of the coating materials varies in composition to produce a heterogeneous coated layer. Also as used herein, composition of the coating material shall also include density of the coating material.
Graded coatings, as defined herein, are used extensively to enhance the mechanical impact resistance, thermal shock resistance, and corrosion resistance of a coating system, as well as for other purposes. Occasionally graded coatings are used to allow thicker coatings to be deposited than would otherwise be possible. Most commonly, the layer of coating next to the substrate is a metallic alloy, and the outermost coating layer is an oxide or cermet. The metallic layer bonds better to the substrate and the oxide or cermet than the cermet or oxide directly to the substrate. It also may improve the mechanical impact resistance and other properties of the total coating by providing a layer of intermediate mechanical properties such as elastic modulus. Other factors such as stress relief through creep of the metallic layer may also be important. An example of this type of system is the use of a nickel based alloy under a tungsten carbide cobalt coating used to repair worn machine components. The thermal shock resistance of a coated system may also be increased with a metallic intermediate layer by increasing the bond strength of the system and providing an intermittent coefficient of thermal expansion between a metallic substrate and an oxide outer coating. This type of coating is frequently used in thermal barrier coating system where the metallic alloy undercoat is also used to protect the substrate from oxidation or other forms of corrosion. A typical example of a thermal barrier coating system, such as aircraft components such as aircraft blades, uses a cobalt-nickel-chromium-aluminum-yttrium metallic alloy undercoat and a zirconia-yttria outer coating. The porosity inherent in the metallic coating is sealed by heat treatment after deposition to provide a corrosion resistant barrier to protect the substrate, frequently a superalloy turbine blade or vane in a gas turbine engine.
In accordance with this invention the properties of many graded coating systems can be improved by increasing the number of layers of coatings with increasing amounts of the material of the final outer layer. This results in a smoother transition in properties as the number of layers increases.
The invention relates to a novel thermal spray process in which a mixture of coating materials is fed to a thermal spray gun or torch and the rate of deposition of the individual components of the mixture varied in a controlled manner by varying the thermal parameters of the thermal spray process gases. As a simple, but not limited example, consider a coating material that consists of two components, A and B, with different properties including melting point, size, shape, heat capacity, and thermal absorption characteristics. The thermal spray deposition parameters may be initially set to optimize the deposition rate or efficiency of A and not B and then changed gradually to optimize the deposition rate or efficiency of B. Thus the deposited coating would have a gradation in composition from predominantly A to A+B to predominately B. In addition to gradations in composition, gradations in other properties such as density can also be made by changing the deposition parameters. While this invention includes coatings that are continuously graded in composition and properties it also includes coatings in which one or more layers of the coating is maintained constant for a given thickness. Any and all of these variations in gradations will be encompassed in the term xe2x80x9cgradedxe2x80x9d as used herein. The coating material is usually fed to the thermal spray device in the form of powder, although one or more of the constituents could be fed in the form of wire or rod. When two or more of the constituents of the coating material are in the form of powder they may be blended mechanically and fed from a single powder dispenser to the thermal spray device or fed individually or in partial blends from two or more powder dispensers to the thermal spray device. The coating material may be fed to the thermal spray device internally as in most detonation gun and high velocity oxy-fuel devices or externally as in many plasma spray devices. The changes in deposition parameters including gas composition and flow rates, power levels, and coating material injection rates may be changed during the deposition process either manually by the equipment operator or automatically by computer control.